A state of art digital hearing aid performs non-linear processing of a converted acoustic signal by adjusting gain as a function of input level and frequency composition of the acoustic signal, within a maximum achievable power output of the hearing aid.
The relationship between the gain and frequency for the hearing aid, generally referred to as the gain transfer function, is determined in accordance with a user's hearing impairment, which is described at least by a minimum detectable sound pressure level and uncomfortable sound pressure level as functions of frequency, and is obviously determined by the maximum power output achievable from the hearing aid. Maximum power output is in this context to be construed as the maximum power at which the hearing aid does not deteriorate or distort the audio signal.
Hence the hearing aid must amplify the acoustic signal so as to provide a sound pressure level of desired/useful sounds to the user above the minimum detectable sound pressure level while avoiding sound pressure levels at the uncomfortable sound pressure level or at the maximum power output. Therefore the gain of the hearing aid is adjusted as a function of the input levels, thus providing dynamic amplification range compression.
In addition, the hearing aid gain is adjusted for suppressing internal input noise from, for example, the microphone, and for suppressing acoustic feedback caused by the proximity of the speaker and microphone.
The terms “attack” and “release” times, which are used hereinafter, are to be construed as the time interval from a sudden increase or decrease of the input level by a predetermined amount in dB until stabilization of the output level from the hearing aid is within +/−2 dB. The attack time is the time required for the hearing aid to initiate an appropriate gaining or dampening process in response to an input change, whereas the release time is the time required for the hearing aid to return to previous operation.
U.S. Pat. No. 4,630,302, which is incorporated by reference in the below specification, discloses a hearing aid apparatus comprising means for differentiating speech signals from typical low level background noise signals present in or derived from microphone means. The apparatus utilizes an automatic gain control amplifier having short attack and long release times, in a manner such that speech signal segments are compressed and have a substantially long time average level. The term “compressed” is in this context to be construed as reduction of gain as a function of increasing input level.
The hearing aid apparatus according to U.S. Pat. No. 4,630,302 further utilizes a noise suppressor having a long attack time (longer than time constants of typical speech segments) and short release time. The noise suppressor is responsive to the output of the automatic gain control amplifier, and has an initiation threshold below the effective specified level of automatic gain control action for the automatic gain control amplifier, say 12 dB. Thus when the output of the automatic gain control amplifier is above the squelch threshold for periods longer than the attack time of the noise suppressor the squelch action is activated, and when the output is below the squelch threshold for a few milliseconds the squelch action is de-activated. Hence speech segments, which typically vary over at least a 12 dB range within periods shorter than the attack time of the noise suppressor, pass through the noise suppressor without being squelched, whereas background noise signal segments which typically present a more steady time average level longer than the attack time of the noise suppressor will be squelched.
However, the long release time of the automatic gain control amplifier disclosed in U.S. Pat. No. 4,630,302 presents a disadvantage in operating a hearing aid in varying sound environments. For example, when a user is exposed to a high sound level, such as caused by the user shouting at a person situated remotely or a door is slammed nearby, the user will be unable to hear low sound levels during a longer period thereafter.
U.S. Pat. No. 6,628,795, which is incorporated by reference in the below specification, discloses a method for automatic, gain control in a hearing aid effected by detecting an input level and/or an output level and adapting the output level in response to the input level by controlling the gain of the hearing aid towards a desired value for the output level. Further, the gain control adjusts attack and release times in response to the detected input level so that a fast gain adjustment is performed at a high input level change and a slow gain adjustment is performed at a low input level change. Thus the gain control provides short attack and release times for input level changes in the high input level area and long attack and release times for input level changes in the low input level area.
However, the method according to U.S. Pat. No. 6,628,795 providing long attack and release times for low level signals introduces a disadvantage in reducing noise, since the user of a hearing aid incorporating this method might loose some speech signals since the release time is long during low input level situations, and therefore in situations where the user moves from a low or no sound situation to a higher level sound situation some information is lost.